Processes for making thin sheets of positive electrodes for lithium ion type electrochemical cells typically consist of mixing active electrode material which is usually in powder form with an electrically conductive material such as carbon or graphite particles and a polymer binder into an organic solvent which is then blended into a homogenized electrode slurry. The electrode slurry is thereafter coated on a current collector sheet in the form of a thin layer or film and the organic solvent is removed from the thin film by heating the thin film to evaporate the solvent. The resulting dried positive electrode thin sheet is typically porous and contains no electrolyte. The dried positive electrode thin sheet is assembled with a separator and a counterpart negative electrode and the assembly is saturated with an ionically conductive liquid electrolyte comprising a lithium salt dissolved therein to form a lithium ion electrochemical cell. The porous positive electrode is filled with the electrolyte to ensure the ionic exchange between the positive and negative electrodes.
When producing large batches or in a continuous process, the large quantity of evaporated solvent which may exceed environmental guide lines may have to be recovered to avoid environmental contamination and the recovery process requires special installations to prevent the solvent in gaseous form from escaping into the environment, and specialty equipments for handling and storing the used solvent which must then be disposed of, or re-used.
Other coating processes for making thin sheets of positive electrode material for solid type lithium based electrochemical cells incorporate into the electrode mixture an electrolyte consisting of a lithium salt solvating polymer and a lithium salt which acts as a binder as well as an electrolyte. The mixture of positive electrode which comprises an active electrode material, an electrically conductive material, a solvating polymer and a lithium salt is blended together with an organic solvent into a homogenized electrode slurry. The electrode slurry is thereafter coated on a current collector sheet in the form of a thin layer or film and the organic solvent is removed from the thin film by heating the thin film to evaporate the solvent. The positive electrode thin sheet thereby produced has little porosity since the electrolyte is already in the electrode and fills the space between the active electrode particles. The positive electrode thin sheet is then assembled with a solid ionically conductive electrolyte sheet consisting of the same or a different polymer and the lithium salt and a counterpart negative electrode to form a lithium based electrochemical cell.
In both cases, the amount of organic solvents is adjusted for diluting the electrode mixture sufficiently in order to lower its viscosity such that the electrode mixture can be spread into thin layers. In both cases, the organic solvents must be removed prior to assembly with the electrolyte separator and the negative electrode to form an electrochemical cell. When producing in large batches or in a continuous process, the organic solvent may have to be recovered and/or treated to avoid discharge into the environment in gaseous form.
US patent application US2006/0166093 discloses a process of making positive and/or negative electrode material in which the processing solvent is water. The disclosed process utilizes a so-called water soluble synthetic rubber (SBR) mixed with a thickening agent as the electrode binder. An aqueous solution adapted for spreading is prepared which include an electrochemically active material, a water soluble synthetic rubber, a thickening agent, optionally an electronically conductive material, and water as the solvent. The aqueous solution of electrode material does not contain lithium salt as the latter is known for its hygroscopic properties which would make it very difficult to dry the electrode after it has been spread into film form in order to remove the water. The film is dried for an extensive period of time (12-24 hours) to reduce the water contain to below 2000 ppm and most preferably below 50 ppm. The electrode sheet produced is porous in order to allow infiltration of a liquid or gelled electrolyte including a lithium salt to provide an ionically conductive path to the particles of electrochemically active material in the electrode. The process disclosed for making an electrode is therefore a two-step process in which the constituents of the electrode excluding the electrolyte are first mixed in water and dried and when the basic electrode is dried, the electrolyte including lithium salt is added to form a working electrode. The process disclosed is therefore specific to lithium ion type electrochemical cells and is not adapted for production of lithium based electrochemical cells having a solid electrolyte which requires that the lithium salt be incorporated in the positive electrode prior to assembly into electrochemical cells.
Thus, there is a need for a method and a process for making electrodes incorporating its ionically conductive electrolyte for lithium based electrochemical cells which is cost effective and environmentally friendly.